Fertility and intelligence

Demographic studies have indicated that in humans, fertility and intelligence tend to be negatively correlated, that is to say, the more intelligent, as measured by IQ, exhibit a lower total fertility rate than the less intelligent. Other correlates of fertility include income, diet, and educational attainment.

Early research

Some of the first studies into the subject were carried out on individuals living before the advent of IQ testing, in the late 19th century, by looking at the fertility of men listed in WHO's WHO, these individuals being presumably of high intelligence. These men, taken as a whole, had few children, implying a negative correlation. [Huntington, E., & Whitney, L. "The Builders of America." New York: Morrow, 1927.] [Kirk, Dudley. 'The fertility of a gifted group: A study of the number of children of men in WHO'S WHO.' In "The Nature and Transmission of the Genetic and Cultural Characteristics of Human Populations." New York: Milbank Memorial Fund, 1957, pp.78-98.]

More rigorous studies carried out on those alive during the Second World War returned different results suggesting a slight positive correlation with respect to intelligence. The findings from these investigations were consistent enough for Osborn and Bajema, writing as late as 1972, to conclude that fertility patterns were eugenic, and that "the reproductive trend toward an increase in the frequency of genes associated with higher IQ... will probably continue in the foreseeable future in the United States and will be found also in other industrial welfare-state democracies." [cite journal
last = Osborn
first = F.
coauthor = Bajema, C.
date = 1972
title = The eugenic hypothesis
journal = Social Biology
volume = 19
pages = 337-345 ] But several reviewers considered the findings premature, arguing that the samples were nationally unrepresentative, generally being confined to whites born between 1910 and 1940 in the Great Lakes States. [cite journal
last = Osborne
first = R.
date = 1975
title = Fertility, IQ and school achievement
journal = Psychological Reports
volume = 37
pages = 1067-1073 ] [cite journal
last = Cattell
first = R. B.
date = 1974
title = Differential fertility and normal selection for IQ: Some required conditions in their investigation
journal = Social Biology
volume = 21
pages = 168-177 ] Other researchers began to report a negative correlation in the 1960s after two decades of neutral or positive fertility. [cite journal
author=Kirk D
date=1969
title=The genetic implications of family planning
journal=Journal of Medical Education
volume=44 (supplement 2)
pages=80-83
url=http://www.popline.org/docs/0082/691265.html ]

In 1982, Daniel Vining sought to address these issues in a large study on the fertility of over 10,000 individuals throughout the United States, who were then aged 25 to 34. The average fertility in his study was correlated at -0.86 with IQ for white women and -0.96 for black women. cite journal
author=Vining Drj
title=On the possibility of the reemergence of a dysgenic trend with respect to intelligence in American fertility differentials
journal=Intelligence
volume=6
issue=3
pages=241–64
year=1982
pmid=12265416] In considering these results along with those from earlier researchers, Vining wrote that "in periods of rising birth rates, persons with higher intelligence tend to have fertility equal to, if not exceeding, that of the population as a whole."

To address the concern that the fertility of this sample could not be considered complete, Vining carried out a follow-up study for the same sample 18 years later, reporting the same, though slightly decreased, negative correlation between IQ and fertility.cite journal
last = Vining
first = Daniel
date = 1995
title = On the possibility of the reemergence of a dysgenic trend with respect to intelligence in American fertility differentials: an update
journal = Personality and Individual Differences
volume = 19
issue = 2
pages = 259–263
url = http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V9F-3YB56P1-2S&_user=521814&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000059575&_version=1&_urlVersion=0&_userid=521814&md5=75ad52e9a7cac14dd14a9acdd31b732d ]

Later research

Regardless of the methodology employed, later research has generally supported that of Vining.

In a 1988 study, Retherford and Sewell examined the association between the measuredintelligence and fertility of over 9,000 high school graduates in Wisconsin in 1957, and confirmed the inverse relationship between IQ and fertility for both sexes, but much more so for females.cite journal
author=Retherford RD, Sewell WH
title=Intelligence and family size reconsidered
journal=Soc Biol
volume=35
issue=1-2
pages=1–40
year=1988
pmid=3217809
url=http://www.ssc.wisc.edu/wlsresearch/publications/files/public/Retherford-Sewell_Intelligence.Family.S.R.pdf
format=PDF]

In a 1999 study Richard Lynn examined the relationship between the intelligence of adults aged 40 and above and their numbers of children and their siblings. Data were collected from the 1994 National Opinion Research Center survey among a representative sample of 2992 English-speaking individuals aged 18 years. Findings revealed that weak negative correlations of -0.05 and -0.09, respectively were found. Further analysis showed that the negative correlation was present only in females. The correlation for females between intelligence and ideal number of children was effectively zero.cite journal
author=Lynn R
title=New evidence for dysgenic fertility for intelligence in the United States
journal=Soc Biol
volume=46
issue=1-2
pages=146–53
year=1999
pmid=10842506]

In 2004 Richard Lynn and Marian Van Court attempted a straightforward replication of Vining's work. Their study returned similar result. [cite journal
author=Lynn R
coauthor=Van Court M
title=New evidence of dysgenic fertility for intelligence in the United States
journal=Intelligence
volume=32
issue=2
pages=193-201
publisher =Ablex Pub.
date= 2004
issn=0160-2896
url=http://eric.ed.gov/ERICWebPortal/custom/portlets/recordDetails/detailmini.jsp?_nfpb=true&_&ERICExtSearch_SearchValue_0=EJ729962&ERICExtSearch_SearchType_0=no&accno=EJ729962]

Fertility and education

Another way of checking the negative relationship between IQ and fertility is to consider the relationship which educational attainment has to fertility, since education is known to be a reasonable proxy for IQ, correlating with IQ at .55; [cite journal
last = Neisser et al.
first = Ulric
coauthors = Boodoo, Gwyneth; Bouchard, Thomas J. Jr.; Boykin, A. Wayde, Brody, Nathan; Ceci, Stephen J.; Halpern, Diane F.; Loehlin, John C.; Perloff, Robert; Sternberg, Robert J.; Urbina, Suzanna
journal = American Psychologist
year = 1996
volume = 51(2)
pages = 77-101
title = Intelligence: Knowns and Unknowns] in a 1999 study examining the relationship between IQ and education in a large national sample, David Rowe and others found not only that achieved education had a high heritability (.68) and that half of the variance in education was explained by an underlying genetic component shared by IQ, education, and SES. [cite journal
last = Rowe
first = David C.
coauthor = Vesterdal, Wendy J. & Rodgers, Joseph L.
date = 1999
title = Herrnstein's Syllogism: Genetic and Shared Environmental Influences on IQ, Education, and Income
journal = Intelligence
volume = 26(4)
pages = 405-423 ] One study investigating fertility and education carried out in 1991 found that high school dropouts in America had the most children (2.5 on average), with high school graduates having fewer children, and college graduates having the fewest children (1.56 on average). [Bachu, Amara. 1991. "Fertility of American Women: June 1990." U.S. Bureau of the Census. Current Population Report Series P-20, No. 454. Washington, D.C.: Government Printing Office.]

Birth control and intelligence

Among a sample of women using a reliable form of birth control, success rates were related to IQ, with the percentages of high, medium and low IQ women having unwanted births during a three-year interval being 3%, 8% and 11%, respectively. [cite journal
first = Richard
last = Urdry
journal = Social Biology
year = 1978
volume = 25
pages = 10-14
title = Differential fertility by intelligence: the role of birth planning] Another study found that after an unwanted pregnancy has occurred, higher IQ couples are more likely to obtain abortions [cite journal
first = Joel
last = Cohen
year = 1971
title = Legal abortions, socioeconomic status and measured intelligence in the United States
journal = Social Biology
volume = 18(1)
pages = 55-63] ; and unmarried teenage girls who become pregnant are found to be more likely to carry their babies to term if they are doing poorly in school. [cite journal
first = Lucy
last = Olson
year = 1980
title = Social and psychological correlates of pregnancy resolution among adolescent women: a review
journal = American Journal of Orthopsychiatry
volume = 50(3)
pages = 432-445] Conversely, while desired family size is apparently the same for women of all IQ levels, [cite journal
last = Vining
first = Daniel
date = 1982
title = On the possibility of the reemergence of a dysgenic trend with respect to intelligence in American fertility differentials
journal = Intelligence
volume = 6
issue = 3
pages = 241-264 ] highly educated women are found to be more likely to say that they desire more children than they have, indicating a "deficit fertility" in the highly intelligent. [cite journal
first = Robert H.
last = Weller
year = 1974
title = Excess and deficit fertility in the United States
journal = Social Biology
volume = 21 (l)
pages = 77-87] In her review of reproductive trends in the United States, Van Court argues that "each factor - from initially employing some form of contraception, to successful implementation of the method, to termination of an accidental pregnancy when it occurs - involves selection against intelligence." [cite journal
first = Marian
last = Van Court
journal = Eugenics Bulletin
year = 1983
title = Unwanted Births And Dysgenic Reproduction In The United States]

International Research

Although much of the research into intelligence and fertility has been restricted to individuals within a single nation (most of them living within the United States), Steven Shatz has recently extended the research internationally; he finds that "There is a strong tendency for countries with lower national IQ scores to have higher fertility rates and for countries with higher national IQ scores to have lower fertility rates." [cite journal
author=Shatz, Steven M.
title=IQ and fertility: A cross-national study
journal = Intelligence
year = 2008
volume = 36
issue = 2
pages = 109-111]

Possible causes

A theory to explain the fertility-intelligence relationship is that fertility is conversely driven by income. [ [http://aladinrc.wrlc.org/dspace/handle/1961/4122 Income as a determinant of declining Russian fertility; Trevitt, Jamie; Public Policy; 18-Apr-2006 ] ] [ [http://www.jstor.org/pss/1809165 The Relation of Economic Status to Fertility; Deborah S. Freedman; The American Economic Review, Vol. 53, No. 3 (Jun., 1963), pp. 414-426] ] This well studied correlation is known as the demographic-economic paradox, which shows an inverse correlation between wealth and fertility within and between nations. The higher the degree of education and GDP per capita of a human population, subpopulation or social stratum, the fewer children are born. In a 1974 UN population conference in Bucharest, Karan Singh, a former minister of population in India, illustrated this trend by stating "Development is the best contraceptive" [cite book |author=David N. Weil |title=Economic Growth |publisher=Addison-Wesley |year=2004 |page=111 |isbn=0201680262]

ee also

*Demographic-economic paradox
*Total fertility rate
*List of countries and territories by fertility rate

Notes